This application is based on Patent Application No. 11-72910 filed in Japan, the content of which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a scanning optical system, for example a scanning optical system used in a film scanner or the like capable of high-speed image retrieval.
2. Description of the Related Art
Various scanning optical systems have been proposed for use in film scanners and the like. Among the proposed systems for high-speed, high-precision image retrieval is a scanning optical system for forming a film image on a unidimensional linear image sensing element (e.g., a line CCD) having an array of photoreceptor elements arranged in a subscan direction via a mirror rotated in a main scan direction to read an object. scanning optical systems suitable for this type of mirror scanning optical system have been disclosed in Japanese Laid-Open Patent Application Nos. 9-236741, 9-236747, 9-236766, and 9-236767.
The scanning optical systems disclosed in these patents use a color separation prism to accomplish so-called 3-panel color separation of color images, and therefore are disadvantageously expensive. Conventionally, in order to reduce cost while maintaining high resolution, it has been thought most effective to eliminate the color separation prism while using a tri-linear image sensing element having the aforesaid unidimensional line image sensing element arrayed in three lines in the main scan direction.
When the aforesaid tri-linear image sensing element is used, however, it becomes necessary not only to correct magnification chromatic aberration, but also axial chromatic aberration which is not a problem in optical systems using the color separation prism. That is, identical focusing is required on the three line image sensing elements on the same surface. When scanning via a mirror, severe disadvantages arise in the scanning optical system from the perspective of the various types of aberration generated, e.g., chromatic aberration, coma and the like.
In view of the previously mentioned disadvantages, an object of the present invention is to provide a high performance scanning optical system at low cost which specifically corrects magnification chromatic aberration and axial chromatic aberration without using a color separation prism, and is not susceptible to performance deterioration when scanning via a mirror.
These objects are attained by a scanning optical system having, an object side lens unit for condensing light from an object, a mirror for performing a main scan to read the image of the object by deflecting the light transmitted through the object side lens; and an image side lens unit for forming an image on the image sensing surface using both the axial light and extra-axial light in a subscan direction deflected by the mirror; and wherein the following condition is satisfied.
0.1 less than |(xcexa3xcexdfpxe2x88x92xcexa3xcexdfm)/Lf| less than 20.0 
Where xcexa3xcexdfp represents the sum of the Abbe numbers of the positive optical power lenses within the object side lens unit, xcexa3xcexdfm represents the sum of the Abbe numbers of the negative optical power lenses within the object side lens unit, and Lf represents the number of lenses in the object side lens unit.